With the recent popularization of computers, inkjet printers are much used for printing on paper, films, cloths and others, not only in office work but also for home use.
The inkjet recoding method includes a system of applying pressure to ink drops by the use of a piezoelectric device to thereby make the ink drops jet out, a system of thermally bubbling ink to jet the ink drops, a system of using ultrasonic waves, and a system of sucking and jetting ink drops by electrostatic force. For the ink compositions for such inkjet recording, usable are aqueous inks, oily inks and solid (hot-melt) inks. Of those, aqueous inks are most used because of their productivity, handlability, odorlessness and safety.
Colorant to be used in such inkjet recording inks must satisfy the following requirements: Its solubility in solvent is good; it enables high-density recording; its hue is good; it is fast to light, heat, air, water and chemicals; it well fixes in image-receiving material and hardly bleeds out; its storage stability in inks is good; it is not toxic; its purity is high; and it is inexpensive and is readily available. However, it is extremely difficult to seek such colorant that satisfies all these requirements on a high level.
Various dyes and pigments have been already proposed for inkjet recording, and are now in practical use. At present, however, no one knows colorant that satisfies all the requirements. Dyes and pigments heretofore well known in the art such as those listed in Color Index (C.I.) could hardly satisfy both the color hue and the fastness, which inks for inkjet recording need.
Having developed inkjet inks that contain dyes, we, the present inventors have found that heterocyclic azo-type-yellow dyes have good weather fastness (for example, JP-A-2004-168865).
However, we have known that, when the dyes are used singly, then there occurs a problem in that the light fastness of the gray area where the dye is mixed with other dyes is not good.
On the other hand, magenta dyes have merits and demerits depending on their type. Concretely, some magenta dyes may have good fastness but cause bleeding in images in high-humidity condition, while some others may have average-level fastness but have a low absorption coefficient per weight and therefore worsen the jet-out stability of inkjet inks. We have known that single dyes could not satisfy all the necessary requirements for inkjet inks.
We have further known that aqueous black inks are problematic in that their image durability is low. Apart from carbon black, there are known few black dyes capable of providing high-quality black images such as letters by themselves, and we have known that some different types of dyes each having a different absorption spectrum must be mixed for forming the intended black images. In this case, however, we have further known that the fading rate may differ between the dyes mixed, and that, when the dyes having abroad absorption characteristic have faded and changed in their absorption wavelength, then the quality of the black-containing image is significantly worsened.
To evade the problem, a method of mixing different types of dyes may be taken into consideration. However, when dyes in a group having the same structure are mixed, then it causes a problem in that a black image having a high reflection density is difficult to obtain.
On the other hand, water-soluble black inks must satisfy the absorption characteristics of covering a broad wavelength range, and are therefore problematic in that their solid concentration is apt to increase and they may worsen the jet-out stability of inks.